Inorganic oxide powder, slurry containing same, nonaqueous electrolyte secondary battery, and method for manufacturing nonaqueous electrolyte secondary battery

ABSTRACT

The present invention relates to an inorganic oxide powder which is suitably used to form an inorganic oxide porous film having excellent heat resistance, insulation properties and film strength, regardless of a small basis weight, and also having porosity capable of imparting sufficient ion permeability on at least one surface of a positive electrode, a negative electrode, or a separator that constitutes a nonaqueous electrolyte secondary battery. Disclosed is an inorganic oxide powder, wherein: 1) an average three-dimensional particle unevenness is 3.6 or more, and 2) an abundance ratio in number of particles having a particle diameter of less than 0.3 μm is 50% or more.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/307,963, filed Oct. 31, 2016, which is a 371 National Stage Entry ofPCT/JP2015/083748, filed Dec. 1, 2015, which claims benefit of priorityto Japanese Patent Application No. 2014-255465, filed Dec. 17, 2014. Theentire disclosures of the prior applications are considered part of thedisclosure of the accompanying divisional application, and are herebyincorporated by reference.

TECHNICAL FIELD

The present invention relates to an inorganic oxide powder which issuitably used to form an inorganic oxide porous film having insulationproperties on at least one surface of a positive electrode, a negativeelectrode or a separator that constitutes a nonaqueous electrolytesecondary battery. The present invention also relates to a slurrycontaining the inorganic oxide powder, and a nonaqueous electrolytesecondary battery including an inorganic oxide porous film containingthe inorganic oxide powder and a method for manufacturing the same.

BACKGROUND ART

A nonaqueous electrolyte secondary battery, especially a lithium ionsecondary battery, has been used in household compact equipment such asa cell phone or a personal computer because of having high energydensity. In recent years, application to automobiles has also beenaccelerated, in addition to the compact equipment.

The nonaqueous electrolyte secondary battery is a battery using anorganic solvent-based electrolytic solution, and usually includes apositive electrode and a negative electrode, and also includes aseparator disposed for the purpose of electrically insulating betweenthese electrode plates. As a separator for a lithium ion secondarybattery, for example, a microporous sheet made of a polyolefin-basedresin is used.

When a short circuit occurs inside a battery, a shutdown function of theseparator made of this microporous sheet leads to closing of pores ofthe separator so as to prevent movement of lithium ions at the shortcircuit part, and accordingly eliminating a battery function at theshort circuit part. In such manner, the separator plays a role inmaintaining safety of the lithium ion secondary battery. However, whenthe temperature of the battery exceeds, for example, 150° C. due tomomentarily generated heat, the separator may drastically contract andthe short circuit part of the positive electrode and the negativeelectrode may expand. In this case, the temperature of the battery mayreach an abnormally overheated state of several hundred degreescentigrade or higher, and therefore, there is a problem in safety.

Patent Document 1 proposes technology of forming an inorganic oxideporous film including an inorganic oxide filler having insulationproperties on a surface of a positive electrode, a negative electrode ora separator that constitutes a lithium ion secondary battery.

PRIOR ART DOCUMENT Patent Document

-   Patent Document 1: JP 9-147916 A

The inorganic oxide porous film disclosed in the Patent Documentmentioned above has high heat resistance and can inhibit drasticcontraction of the separator.

However, even when an inorganic oxide porous film is formed using aninorganic oxide powder satisfying various physical properties disclosedin the Patent Document, the inorganic oxide porous film thus obtainedhas insufficient average pore radius or porosity and lacks in ionpermeability, thus resulting in a problem that a nonaqueous electrolytesecondary battery such as a lithium ion secondary battery including theinorganic oxide porous film exhibits insufficient load characteristics.

In recent years, application to automobile use and the like with highdensity and high output capacity has been accelerated, and there is aneed to thin each constituent material of a nonaqueous electrolytesecondary battery. There is also a need to ensure safety and to maintainappropriate battery performance, even when an inorganic oxide porousfilm is further thinned, namely, a basis weight of an inorganic oxide isreduced. However, there is an actualized problem which is that onlysimple reduction in a basis weight of an inorganic oxide lead to furtherreduction in an amount of the inorganic oxide existing on a surface, sothat an advantage such as high heat resistance as a feature of theinorganic oxide is not sufficiently exhibited, or the heat resistance isdrastically impaired, thus the safety cannot sufficiently be ensured.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

Under these circumstances, an object of the present invention is toprovide an inorganic oxide powder which is suitably used to form aninorganic oxide porous film having excellent heat resistance, insulationproperties and film strength, regardless of a small basis weight, andalso having porosity capable of imparting sufficient ion permeability onat least one surface of a positive electrode, a negative electrode or aseparator that constitutes a nonaqueous electrolyte secondary batterytypified by a lithium ion secondary battery.

Means for Solving the Problems

The inventors has intensively studied so as to solve the problemsmentioned above, thus reaching to finely granulating of an inorganicoxide powder included in an inorganic oxide porous film whilemaintaining a special shape (three-dimensional particle unevenness) ofthe inorganic oxide powder.

Namely, the present invention includes the following inventions.

<1> An inorganic oxide powder, wherein:

1) an average three-dimensional particle unevenness is 3.6 or more, and

2) an abundance ratio in number of particles having a particle diameterof less than 0.3 μm is 50% or more.

<2> The inorganic oxide powder according to <1>, wherein the powder hasa BET specific surface area is 6.0 m²/g or more.<3> The inorganic oxide powder according to <1> or <2>, wherein theinorganic oxide is α alumina.<4> An inorganic oxide slurry, including: the inorganic oxide powderaccording to any one of <1> to <3>; a binder; and a solvent.<5> A nonaqueous electrolyte secondary battery, wherein: an inorganicoxide porous film is formed on at least one surface of a positiveelectrode, a negative electrode or a separator, the film havinginsulation properties and, including the inorganic oxide powderaccording to any one of <1> to <3>.<6> The nonaqueous electrolyte secondary battery according to <5>,wherein, in the inorganic oxide porous film, a proportion of a total ofa pore volume inside coating film of pores inside coating film having apore diameter inside coating film of 0.2 μm or less, relative to a totalof a pore volume inside coating film of all pores inside coating film,is 35% or more.<7> A method for manufacturing a nonaqueous electrolyte secondarybattery, including: a step of applying the inorganic oxide slurryaccording to <4> on at least one surface of the positive electrode, thenegative electrode or the separator, followed by drying the slurry toform an inorganic oxide porous film.

Effects of the Invention

According to the present invention, there is provided an inorganic oxidepowder which is suitable to form an inorganic oxide porous film havingexcellent heat resistance, insulation properties, and film strength,regardless of a small basis weight, and also having porosity capable ofimparting sufficient ion permeability. The inorganic oxide porous filmformed of the inorganic oxide powder is excellent in loadcharacteristics because of having excellent ion permeability, and alsohas high heat resistance and film strength, so that a nonaqueouselectrolyte secondary battery including the inorganic oxide porous filmon at least one surface of a positive electrode, a negative electrode ora separator is a secondary battery which can simultaneously achieve bothbattery performance and safety.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram for describing a three-dimensionalparticle unevenness.

FIG. 2 is a schematic diagram for describing a pore diameter insidecoating film, and a pore volume inside coating film.

MODE FOR CARRYING OUT THE INVENTION

The present invention will be described in detail below. In the presentdescription, when the expression “to” is used for a range of a value,the range contains the upper and lower limits.

The present invention is directed to an inorganic oxide, wherein:

1) an average three-dimensional particle unevenness is 3.6 or more, and

2) an abundance ratio in number of particles having a particle diameterof less than 0.3 μm is 50% or more (hereinafter sometimes referred to asan “inorganic oxide powder of the present invention”, or simply referredto as an “inorganic oxide powder”).

The inorganic oxide powder of the present invention is not particularlylimited as long as it is a material that has electrically insulationproperties, and aluminum oxide, titanium oxide, magnesium oxide and thelike can be used as an oxide component thereof. These oxides may be usedalone, or two or more oxides may be used as a mixture.

Among these, aluminum oxide (alumina) is preferable, and α alumina,which is excellent in insulation properties and heat resistance and ischemically stable, is particularly preferable.

One of features of the inorganic oxide powder of the present inventionis that inorganic oxide particles constituting the inorganic oxidepowder have a predetermined shape (three-dimensional particleunevenness) and size.

Here, the “three-dimensional particle unevenness” is a shape parameterof one inorganic oxide particle constituting the inorganic oxide powder,which is the value defined by the following formula (1), based on aparticle volume V (μm³) and a volume of rectangular parallelepipedLa×Lb×Lc (μm³) circumscribing the particles.

Three-dimensional particle unevenness=La×Lb×Lc/V   (1)

where La denotes a major diameter of particle, Lb denotes a middlediameter of particle, and Lc denotes a minor diameter of particle, andLa, Lb and Lc are orthogonal to one another. FIG. 1 shows a schematicdiagram for describing the three-dimensional particle unevenness. The“average three-dimensional particle unevenness” can be obtained as anindex showing a feature of the particle shape by calculating thethree-dimensional particle unevenness from 100 or more particles usingthe above formula (1). The average three-dimensional unevenness as usedherein is the value of the three-dimensional particle unevennesscorresponding to a cumulative volume of 50% calculated when cumulativevolume distribution to the three-dimensional particle unevenness isdetermined for arbitrary particles of 100 or more.

One of features of the inorganic oxide powder of the present inventionis also that the powder contains many fine particles having a particlediameter of less than 0.3 μm while having a large unevenness. Here, the“particle diameter” is one of parameters of inorganic oxide particles,which means a diameter d of a sphere having the volume which is equal tothe particle volume V (μm³) of the inorganic oxide powder, and is thevalue satisfying the following formula (2).

V=4π/3×(d/2)³  (2)

An abundance ratio in number of particles having a particle diameter ofless than 0.3 μm can be obtained by calculating the “particle diameter”from 100 or more particles using the above formula (2).

The particle volume V, the major diameter La of particle, the middlediameter Lb of particle, the minor diameter Lc of particle, and thediameter d of sphere can be obtained by analyzing continuous sliceimages of the particles to be targeted using the three-dimensionalquantitative analysis software (for example, TRI/3D-PRT manufactured byRATOC SYSTEM ENGINEERNING CO., LTD.).

The continuous slice images of the particles can be obtained in thefollowing manner. That is, a sample for evaluation obtained by curing aparticle fixing resin (epoxy resin and the like) containing apredetermined amount of the inorganic oxide powder dispersed therein issliced by the FIB processing at specific intervals to obtain thepredetermined number of continuous cross-sectional SEM images byrepeatedly obtaining the cross-sectional SEM images. The obtainedcross-sectional SEM images are subjected to position correction usingappropriate image analysis software (for example, Avizo ver 6.0manufactured by Visualization Sciences Group), and then the obtainedcontinuous slice images are subjected to three-dimensional quantitativeanalysis.

A specific evaluation procedure of the three-dimensional particleunevenness and particle diameter (sample preparing method for continuousslice images, and calculation method for V, La, Lb, Lc and d using athree-dimensional quantitative analysis software) will be mentioned indetail in Examples, taking an alumina particle as an example.

The average three-dimensional particle unevenness of the inorganic oxidepowder of the present invention defined by the above method ischaracterized by being 3.6 or more, preferably 3.8 or more, and morepreferably 4.0 or more. The upper limit of the average three-dimensionalparticle unevenness is preferably 10.0 or less, and more preferably 6.0or less.

When the average three-dimensional particle unevenness is adjusted to3.6 or more, it is possible to improve porosity and ion permeability ofthe inorganic oxide porous film which is obtained by slurrying theinorganic oxide powder and applying the slurried inorganic oxide powderon a surface of a separator or that of an electrode (positive electrodeor negative electrode) made of an electrode mixture layer containing anelectrode active material (positive electrode active material ornegative electrode active material) and a binder, followed by drying.Taking the porosity of the inorganic oxide porous film and strengththereof into consideration, the average three-dimensional particleunevenness is preferably 10 or less.

The inorganic oxide powder of the present invention is characterized byincluding 50% or more of particles having a particle diameter of lessthan 0.3 μm defined by the above-mentioned method while having a largeunevenness relative to the number of all particles constituting theinorganic oxide powder (abundance ratio in number of particles having aparticle diameter of less than 0.3 μm), and preferably 55% or more, morepreferably 60% or more, and most preferably 65% or more. The upper limitis not particularly limited and may be 100%. When the powder includesfine particles having a large three-dimensional particle unevenness andthe above-mentioned proportion, it is possible to maintain an optimumrange of the porosity of the inorganic oxide porous film which isobtained by slurrying the inorganic oxide powder and applying theslurried inorganic oxide powder on a surface of a separator or that ofan electrode mixture layer containing an electrode active material and abinder, followed by drying, thus leading to good ion permeability andelectrolytic solution retention performance of the inorganic oxideporous film. The inorganic oxide porous film made of such particlesexhibits high film strength since a contact point between particlesincreases and a strong three-dimensional network can be formed whilemaintaining the porosity, and for example heat resistance anddimensional stability of the separator are improved due to reduction inpowder falling of the inorganic oxide, and thus a nonaqueous electrolytesecondary battery having higher safety is obtained.

The oxide purity of the inorganic oxide powder of the present inventionis usually 99% by weight or more, preferably 99.9% by weight or more,and more preferably 99.99% by weight or more.

The “oxide purity” means the proportion of the oxide component composedof aluminum oxide, titanium oxide, magnesium oxide and the like, or amixture thereof when the total weight of all components in the inorganicoxide powder of the present invention is regarded as 100% by weight. Themeasurement method will be mentioned below in Examples taking an examplewhere a normative oxide component is α alumina.

In particular, when the inorganic oxide powder of the present inventionis especially an α alumina powder, for example in battery application,it is not preferable that the purity is less than 99% by weight for thefollowing reason. That is, impurities such as Si, Na or Fe contained inthe α alumina powder increase, as a result, good electric insulationproperties cannot be obtained, furthermore, mixed amount of metallicforeign materials causing a short circuit increases.

The BET specific surface area of the inorganic oxide powder of thepresent invention is preferably 6.0 m²/g or more, more preferably 6.5m²/g or more, and most preferably 7.0 m²/g or more. When the BETspecific surface area is within the above range, connection with thebinder is improved when an inorganic oxide porous film is formed by thebelow-mentioned method, thus an inorganic oxide porous film having highstrength is obtained.

The oxide component of the inorganic oxide powder of the presentinvention is preferably alumina, and particularly preferably α alumina.When the inorganic oxide powder of the present invention is α alumina, acoating film can be obtained by mixing an α alumina powder, a binder anda solvent to prepare an α alumina slurry, and applying the α aluminaslurry on a surface of a separator or that of a positive electrode or anegative electrode made of an electrode mixture layer containing anelectrode active material. A consolidation processing such as rollingmay also be performed, whereby, it is possible to sufficiently ensureporosity of the α alumina porous film suited for ion conduction, andsimultaneously, the porosity can be arbitrarily controlled within apreferable range.

A method for producing an α alumina powder suitable as the inorganicoxide powder of the present invention is not particularly limited,examples of the method for producing an α alumina powder include; amethod of calcining aluminum hydroxide prepared by an aluminum alkoxidemethod; a synthesis method using organoaluminum; a method of calciningtransition alumina, or an alumina powder which is converted intotransition alumina by subjecting to a heat treatment, as a raw material,in an atmospheric gas containing hydrogen chloride; a method disclosedin JP 2010-150090 A, JP 2008-100903 A, JP 2002-047009 A or JP2001-354413 A; and the like.

The aluminum alkoxide method includes, for example, a method ofhydrolyzing an aluminum alkoxide with water to give a slurry-, sol-, orgel-like aluminum hydroxide, followed by drying to obtain a dry-powderedaluminum hydroxide and the like.

The powdered aluminum hydroxide obtained by drying is a bulky powderwhich usually has an untamped density within a range of about 0.1 to 0.4g/cm³, and preferably 0.1 to 0.2 g/cm³. The powdered aluminum hydroxideis not limited thereto, and the thus obtained aluminum hydroxide powdermay be used after adjusting to an arbitrary bulk density by making thebulk density higher at post-process and the like.

A cumulative pore volume (pore radius is within a range of 0.01 μm ormore and 1 μm or less) of aluminum hydroxide is not particularlylimited, and is preferably 0.6 mL/g or more. In this case, because ofsmall primary particle, excellent dispersibility, and less agglomeratedparticles, an alumina calcined body obtained by calcination can preventthe generation of alumina agglomerated particles that are stronglycoupled and are hard to crush.

A method for measuring the pore volume is as follows.

A sample devoted to the measurement is dried by a dryer at a temperatureof 120° C. for 4 hours and the sample after drying is precisely weighfor taking it as sample weight.

The sample after drying is set to a cell of the pore volume measuringdevice (“Auto Pore 1119420” manufactured by MICROMERITICS), and mercuryis filled within the system after inside the cell system is droppedbelow 50 μmHg. Then, pressure is gradually applied to the cell from0.007 MPa to 414 MPa to measure the amount of mercury penetration, undereach pressure, by setting penetration equilibrium waiting time ofmercury at 10 seconds.

The cumulative pore volume (mL/g) is determined by dividing the totalamount of mercury intrusion (mL) at the time when pressure is appliedfrom 0.007 MPa to 414 MPa by the sample weight (g).

The objective α alumina powder can be obtained by calcining thedry-powdered aluminum hydroxide obtained by the aluminum alkoxidemethod.

The aluminum hydroxide is usually calcined in a state of being filledinto a calcination container. The calcination container includes, forexample, sheath, saggar and the like.

A material of the calcination container is preferably alumina from theviewpoint of the prevention of contamination of the thus obtained αalumina powder, and particularly preferably high-purity α alumina. Fromthe viewpoint of heat resistance and usage cycle characteristic of thecalcination container, a material containing silica or a magnesiacomponent and the like in an appropriate range may be used.

A method for filling the aluminum hydroxide into the calcinationcontainer is not particularly limited, and the aluminum hydroxide may befilled into the calcination container by the self-weight, or filledafter consolidation.

Examples of the calcination furnace to be used for calcination of thealuminum hydroxide include a material stationary-type calcinationfurnaces typified by a tunnel kiln, a batch-type air flow-type box typecalcination furnace or a batch-type air co-flow box-type calcinationfurnace and the like, a rotary kiln or an electric furnace and the like.

The calcination temperature of the aluminum hydroxide, the temperaturerise rate to the calcination temperature, and the calcination time areappropriately selected so as to obtain a alumina having desired physicalproperties.

The calcination temperature of the aluminum hydroxide is, for example,1,000° C. or higher and 1,450° C. or lower, and preferably 1,000° C. orhigher and 1,350° C. or lower. The temperature rise rate when thetemperature is raised to this calcination temperature is usually 30°C./hour or more and 500° C./hour or less. The calcination times of thealuminum hydroxide is usually 0.5 hour or more and within 24 hours, andpreferably 1 hour or more and within 20 hours.

The aluminum hydroxide may be calcined, for example, in an airatmosphere, or in an inert gas atmosphere such as a nitrogen gas orargon gas, or may be calcined in an atmosphere with high water vaporpartial pressure, as in a gas furnace for calcination by burning of apropane gas and the like. Usually, when calcined in an atmosphere withhigh water vapor partial pressure, the thus obtained particles areeasily baked and densified by the effect of water vapor, unlikecalcination in the air atmosphere.

The α alumina powder thus obtained after calcination is sometimesagglomerated in a state where the average particle diameter exceeds 10μm. In that case, it is preferred to crush the powder as long as theshape of particles is not impaired.

In that case, the powder can be crushed using, for example, knowndevices such as a vibration mill and a jet mill, and it is possible touse either a method of crushing in a dry state or a method of crushingin a wet state. However, in the case of crushing using media such as aceramic ball, there arises problems such as inclusion of a mediaabrasion powder, contamination of an inorganic oxide powder withimpurities due to contact between media and the inorganic oxide powder,or degradation of unevenness of an inorganic oxide powder due tocollision between media and the inorganic oxide powder. Therefore, it ispreferred to perform medialess crushing. When the powder is crushed in adry state, known auxiliary agents may be added for the purpose ofimproving productivity.

The inorganic oxide powder of the present invention may be subjected toa surface treatment and the like. Examples of the method of a surfacetreatment include, but are not limited to, a method of using a surfacetreatment agent such as a coupling agent or a surfactant. The couplingagent may have a functional group such as an amino group, an epoxygroup, or an isocyanate group in a molecular structure thereof. Thesurface treatment of the inorganic oxide powder with a coupling agenthaving these functional groups exerts the effect such as improvement ofbinding with a binder, and improvement of dispersibility of theinorganic oxide powder in the below-mentioned inorganic oxide slurry.

The inorganic oxide slurry of the present invention includes theabove-mentioned inorganic oxide powder of the present invention, binderand solvent.

It is possible to use known binders, for example, fluororesins such aspolyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE) and atetrafluoroethylene-hexafluoropropylene copolymer (FEP); polyacrylicacid derivatives such as polyacrylic acid, polymethyl acrylate,polyethyl acrylate and polyhexyl acrylate; polymethacrylic acidderivatives such as polymethacrylic acid, polymethyl methacrylate,polyethyl methacrylate and polyhexyl methacrylate; polyamide, polyimide,polyamideimide, polyvinyl acetate, polyvinylpyrrolidone, polyether,polyethersulfone, hexafluoropolypropylene, a styrene-butadiene rubber,carboxymethyl cellulose (hereinafter referred to as CMC),polyacrylonitrile, and derivatives thereof, polyethylene, polypropylene,an aramid resin and the like, or salts thereof. These binders may beused alone, or two or more binders may be used as a mixture.

It is also possible to use a copolymer of two or more materials selectedfrom tetrafluoroethylene, hexafluoroethylene, hexafluoropropylene,perfluoroalkyl vinyl ether, vinylidene fluoride,chlorotrifluoroethylene, ethylene, propylene, pentafluoropropylene,fluoromethyl vinyl ether, acrylic acid, and hexadiene.

It is possible to use known solvents, for example, water, alcohol,acetone, tetrahydrofuran, methylene chloride, chloroform,dimethylformamide, N-methyl-2-pyrrolidone (NMP), cyclohexane, xylene,cyclohexanone or mixed solvents thereof.

The content of the binder in the inorganic oxide slurry of the presentinvention is not particularly limited, for example, and is preferablywithin a range of 0.1 to 20 parts by weight relative to 100 parts byweight of the inorganic oxide powder of the present invention. Thecontent of the solvent in the inorganic oxide slurry of the presentinvention is not particularly limited, for example, and is preferablywithin a range of 10 to 500 parts by weight relative to 100 parts byweight of the inorganic oxide powder of the present invention.

Various additives having a function such as dispersants, thickeners,leveling agents, antioxidants, defoamers, pH adjustors containing anacid or alkali, and additives having a function such as inhibition ofside reaction such as decomposition of an electrolytic solution may beadded to the inorganic oxide slurry of the present invention, inaddition to the above-mentioned components, for the purpose ofdispersion stabilization or improvement in coatability. These additivesare not particularly limited as long as they are chemically stablewithin a range of use of the nonaqueous electrolyte secondary battery,and do not exert a large influence on a battery reaction. These variousadditives are preferably those capable of being removed when theinorganic oxide porous film is formed, but may remain in the porousfilm. The content of each additive is not particularly limited, forexample, and is preferably 10 parts by weight or less relative to 100parts by weight of the inorganic oxide powder of the present invention.

The inorganic oxide slurry of the present invention can be prepared bymixing the inorganic oxide powder of the present invention, a binder,and a solvent, followed by dispersion. A method for dispersing theinorganic oxide slurry is not particularly limited, and it is possibleto use a stirring method using a known planetary mixer and the like, ora dispersing method using ultrasonic irradiation.

The inorganic oxide porous film produced from the thus obtainedinorganic oxide slurry has high heat resistance and exhibits insulationproperties. This inorganic oxide porous film is formed on at least onesurface of a positive electrode, a negative electrode or a separator,and is suitably used in a nonaqueous electrolyte secondary batteryincluding an electrode group (laminated type electrode group) formed bylaminating together with a positive electrode, a negative electrode anda separator, or an electrode group (wound type electrode group) formedby laminating the inorganic oxide porous film together with a positiveelectrode, a negative electrode and a separator and then winding, and anelectrolytic solution.

The method for suitably manufacturing such nonaqueous electrolytesecondary battery includes a method including the step of applying theabove-mentioned inorganic oxide slurry on a surface of a positiveelectrode and/or a negative electrode made of an electrode mixture layercontaining an electrode active material (positive electrode activematerial or negative electrode active material) and a binder, followedby drying the slurry to form an inorganic oxide porous film. The methodmay also be a manufacturing method including the step of applying theabove-mentioned inorganic oxide slurry on a separator, in place of asurface of a positive electrode and/or a negative electrode, followed bydrying the slurry to form an inorganic oxide porous film.

More specific manufacturing method, for example, a method formanufacturing a nonaqueous electrolyte secondary battery including awound type electrode group in which an inorganic oxide porous film isformed on a negative electrode includes a method in which one end of anegative electrode lead is joined to the negative electrode lead joiningpart imparted with an inorganic oxide porous film on a surface and theother end of the negative electrode lead is joined to a positiveelectrode lead joining portion, and the positive electrode and thenegative electrode are laminated via a separator and wound to form awound type electrode group, and then this electrode group is stored in abattery can in a state of that the group is interposed between upper andlower insulation rings, followed by injection of an electrolyticsolution and further sealing with a battery cap.

There is no particular limitation on a method for applying the inorganicoxide slurry on a surface of an electrode mixture layer containing apositive electrode active material or negative electrode active materialand a binder or a surface of a separator surface and, for example, aknown doctor blade method or a gravure printing method and the like canbe used. There is also no particular limitation on a drying method, anda known hot-air drying or vacuum drying method and the like can be used.The thickness of the inorganic oxide porous film obtained in that caseis preferably within a range of about 0.3 to 20 μm, and more preferablyabout 0.5 to 10 μm.

There is no particular limitation on battery constituent materials of apositive electrode, a negative electrode, a separator, an electrolyticsolution and the like of a nonaqueous electrolyte secondary battery, andconventionally known materials can be used. For example, materialsdisclosed in known document such as WO 09/041722 A can be used.

The nonaqueous electrolyte secondary battery of the present inventionmanufactured by the above-mentioned manufacturing method includes aninorganic oxide porous film made of the inorganic oxide powder of thepresent invention.

One of features of the inorganic oxide porous film made of the inorganicoxide powder of the present invention is that fine pores are includedinside coating film. In the present description, the “inorganic oxideporous film” is sometimes referred to as a “coating film”.

Here, a property of the inorganic oxide porous film (coating film) canbe expressed by a “pore diameter inside coating film” and a “pore volumeinside coating film” described below as parameters. A schematic diagramfor describing a pore diameter inside coating film, and a pore volumeinside coating film is shown in FIG. 2.

The “pore diameter inside coating film” is one of parameters which canbe determined from three-dimensional analysis of the inorganic oxideporous film in the same manner as in three-dimensional particleunevenness. When a region between branching points of a void shown inFIG. 2 is regarded as one pore and this is defined as a pore insidecoating film (hereinafter sometimes referred simply to as a “pore”), the“pore diameter inside coating film” is the value obtained by dividingthe sum of a minor diameter (Thickness) and a major diameter (Width) ofthe pore shown in FIG. 2 by 2, which is defined by the following formula(3).

Pore diameter inside coating film=(Thickness+Width)/2   (3)

In the case of determining the pore diameter inside coating film, theparticle portion and the void portion are distinguished by binarizing aninorganic oxide porous film using three-dimensional analysis. Withrespect to the distinguished void portion, line thinning processing isperformed on software, and a linkage point of three or more networks ornetworks each having a different width is regarded as a branching pointof a pore inside coating film. With respect to pores inside coating filmbetween all branching points, a minor diameter (Thickness), a majordiameter (Width) and a distance between branching points (Length) arecalculated.

The pore volume inside coating film (hereinafter sometimes referredsimply to as a “pore volume”) is the value which is obtained bycalculating a cross-sectional area (CS) of pores from a minor diameter(Thickness) and a major diameter (Width) by the following formula (4),and defining from the thus obtained cross-sectional area and pore length(distance between branching points (Length)) by the following formula(5).

CS=(Thickness/2)×(Width/2)×π  (4)

Pore volume inside coating film=CS×Length  (5)

It is also possible to determine pore distribution inside coating filmin the coating film from the thus obtained pore diameter and pore volumein pores inside coating film, and to determine a volume proportion ofpores having a pore diameter within a specific range.

In order that the inorganic oxide porous film according to the presentinvention has more excellent heat resistance, insulation properties andfilm strength, a volume proportion of pores having a pore diameter of0.2 μm or less are preferably larger, among pores inside coating film.More specifically, when the volume proportion of pores having a porediameter of 0.2 μm or less is defined by the proportion of a total of apore volume of pores having a pore diameter of 0.2 μm or less relativeto a total of a pore volume of all pores inside coating film in theinorganic oxide porous film (coating film) according to the presentinvention ((“total of a pore volume of pores having a pore diameter of0.2 μm or less”/“total of a pore volume of all pores inside coatingfilm”), the proportion is preferably 35% or more, more preferably 40% ormore, and most preferably 50% or more (including 100%).

When the volume proportion of pores having a pore diameter of 0.2 μm orless satisfies the above requirements, the inorganic oxide porous filmaccording to the present invention has more excellent heat resistance,insulation properties and film strength. Therefore, a nonaqueouselectrolyte secondary battery including such the inorganic oxide porousfilm is excellent in heat resistance and dimensional stability at theshutdown temperature of a separator, leading to more excellent safety.

A specific evaluation procedure of the pore diameter inside coating filmand pore volume inside coating film (sample preparing method forcontinuous slice images, and calculation method for each value using athree-dimensional quantitative analysis software) will be mentioned indetail in Examples, taking an inorganic oxide porous film made of analumina coating film as an example.

It is also one of features that the inorganic oxide porous film made ofthe inorganic oxide powder of the present invention has a sufficientporosity. In the inorganic oxide porous film according to the presentinvention, the porosity defined below is preferably within a range of 30to 75%, and more preferably 35 to 70%.

When the porosity satisfies the above requirements, the inorganic oxideporous film according to the present invention has more excellent ionpermeability. Therefore, a nonaqueous electrolyte secondary batteryincluding such the inorganic oxide porous film is excellent in ionpermeability.

Here, the “porosity” in the present invention is a parameter indicatinga void inside the inorganic oxide porous film, and can be determined bythree-dimensional analysis of the inorganic oxide porous film in ananalysis region. The porosity is the value obtained by dividing thetotal volume (BV) of the void portion by the total volume (TV) in theanalysis region, with respect to the void portion distinguished bybinarizing into the particle portion and the void portion usingthree-dimensional analysis, which is defined by the following formula(6):

Porosity=BV/TV  (6)

A specific evaluation procedure of the porosity (sample preparing methodfor continuous slice images, and calculation method for each value byusing a three-dimensional quantitative analysis software) will bedetailed in Examples, taking an inorganic oxide porous film made of analumina coating film as an example.

As mentioned above, when the volume proportion of pores or porositysatisfies the above requirements in the inorganic oxide porous filmaccording to the present invention, because of having high ionpermeability while having more excellent heat resistance, insulationproperties and film strength, a nonaqueous electrolyte secondary batteryincluding such the inorganic oxide porous film can simultaneouslyachieve both safety and battery performance, and is excellent.

In the case of a laminated porous film in which this inorganic oxideporous film is formed, for example, on a separator, air permeability isoften used as a battery performance and is usually indicated by theGurley value. The Gurley value indicated by the number of seconds thatan air permeates from one face to the other face, and is preferablywithin a range of 30 to 1,000 seconds/100 cc, more preferably 50 to 500seconds/100 cc, and most preferably 50 to 350 seconds/100 cc, though theGurley value of the laminated porous film varies depending on a basematerial porous film as a separator.

In the case of a laminated porous film in which this inorganic oxideporous film is formed, for example, on a separator, dimensionalstability of the laminated porous film in a high temperature range atwhich shutdown occurs is often used as evaluation of safety, and isusually indicated by a heat shape retention ratio. In general, the heatshape retention ratio of the laminated porous film is preferably 80% ormore, more preferably 85% or more, and most preferably 90% or more. Thehigh temperature range at which shutdown occurs as used herein is withina range of 80 to 180° C., and may refer to a rage of about 130 to 170°C.

EXAMPLES

The present invention will be described in detail by way of Examples,but the present invention is not limited only to the following Examples.Methods for evaluation of the respective physical properties are asfollows.

(Oxide Purity)

Oxide purity (% by weight) of an inorganic oxide powder was determinedfrom the following calculation formula by regarding the total of theweight of oxide (α alumina) as a reference, and the sum of the weightsof SiO₂, Na₂O, MgO, CuO, Fe₂O₃, and ZrO₂ included in the oxide as thereference as 100 (% by weight). SiO₂, Na₂O, MgO, CuO, Fe₂O₃ and ZrO₂ aredefined as impurities to oxide (a alumina) as the reference.

Oxide purity (% by weight)=100−sum of weights of impurities (% byweight)

The weights of SiO₂, Na₂O, MgO, CuO and Fe₂O₃ as impurities weredetermined by converting the contents of Si, Na, Mg, Cu, and Fe, whichare obtained by measuring a sample for evaluation using solid-stateemission spectrometry, and the weight of ZrO₂ as the remaining impuritywas determined by converting the content of Zr, which is obtained bymeasuring a sample for evaluation using ICP emission spectroscopy, intothe weights of oxides (SiO₂, Na₂O, MgO, CuO, Fe₂O₃, ZrO₂) correspondingto each element.

(BET Specific Surface Area)

Using “Flow Sorb II 2300” manufactured by Shimadzu Corporation as aspecific surface area measuring apparatus, a BET specific surface areawas determined by the nitrogen absorption method (one point method) inaccordance with the method defined in JIS-Z8830(2013).

(Average Three-Dimensional Particle unevenness, Particle Diameter)

2 parts by weight of a dispersant and 2 parts by weight of an aluminaparticle powder were dispersed into 100 parts by weight of an epoxyresin. After vacuum deaeration, 12 parts by weight of a curing agent wasadded and the thus obtained alumina dispersed epoxy resin was pouredinto a silicon mold, followed by curing.

After a cured sample was fixed to a sample stage, Pt—Pd wasvacuum-deposited and the sample was set to FIB-SEM (HELIOS 600manufactured by FEI). The FIB processing was applied to the sample at anaccelerating voltage of 30 kV to form a cross section, and the crosssection was observed by SEM at an accelerating voltage of 2.1 kV. Afterobservation, the FIB processing was again applied to the sample with athickness of 20 nm in a depth direction of the sample to form a newcross section, and the cross section was observed by SEM. In thismanner, the FIB processing and SEM observation of the cross section wererepeated at intervals of 20 nm to obtain continuous images more than 100images. A position correction was made by the image analysis software(Avizo ver. 6.0 manufactured by Visualization Sciences Group) to obtaincontinuous slice images. The scale was set to 19 nm/pix for X-axis andY-axis, and 20 nm/pix for Z-axis.

A three-dimensional analysis of the alumina particles was applied to theobtained continuous slice images to calculate a three-dimensionalparticle unevenness and a particle diameter. For the three-dimensionalquantitative analysis, the quantitative analysis software TRI/3D-PRT(manufactured by RATOC SYSTEM ENGINEERING CO., LTD.) was used.

The three-dimensional quantitative analysis was made as follows. Thatis, files of the continuous slice images were firstly open on theTRI/3D-PRT, noises are removed by applying a median filter.Subsequently, three-dimensionally isolated particles are respectivelydistinguished and labeled, and then particles which are cut by acircumference of a measuring area were removed.

A particle volume V of an arbitrary particle, a major diameter La, amiddle diameter Lb and a minor diameter Lc of the particle weredetermined from 100 or more particles which were left unremoved in theabove processing. A particle diameter d and a three-dimensional particleunevenness were calculated from the above formulas (1) and (2). Theaverage three-dimensional particle unevenness was calculated, excludingparticles having a particle diameter of less than 0.3 μm and more than 1μm. Namely, the average three-dimensional particle unevenness wasdetermined as the value of particles having a particle diameter of 0.3μm or more and 1 μm or less.

(Preparation of Base Material Porous Film (Separator))

70% by weight of an ultra-high molecular weight polyethylene powder(340M, manufactured by Mitsui Chemicals, Inc.) and 30% by weight of apolyethylene wax having a weight average molecular weight of 1,000(FNP-0115, manufactured by Nippon Seiro Co., Ltd.) were added, and 0.4part by weight of an antioxidant (Irg1010, manufactured by CibaSpecialty Chemicals Inc.), 0.1 part by weigh of an antioxidant (P168,manufactured by Ciba Specialty Chemicals Inc.), and 1.3 parts by weightof sodium stearate were added relative to 100 parts by weight of thetotal amount of the ultra-high molecular weight polyethylene and thepolyethylene wax, and then calcium carbonate having an average particlediameter of 0.1 μm (manufactured by Maruo Calcium Co., Ltd.) was addedso that the proportion became 38% by volume relative to the totalvolume. After mixing them in the form of a powder using a Henschelmixer, the thus obtained powder mixture was melt-kneaded by a twin-screwkneader to obtain a polyolefin resin composition. The polyolefin resincomposition was rolled by a pair of rolls at a surface temperature of150° C. to form a sheet. This sheet was immersed in an aqueoushydrochloric acid solution (hydrochloric acid of 4 mol/L, nonionicsurfactant of 0.5% by weight) to remove calcium carbonate. Subsequently,the sheet was stretched by six times at 105° C. to obtain a basematerial porous film (film thickness: 16.2 μm, basis weight: 7.3 g/m²,air permeability: 140 seconds/100 cc).

(Preparation of Laminated Porous Film for Evaluation)

A laminated porous film for evaluation was formed as a sample film forevaluation of an inorganic oxide porous film by the following method.

CMC; part number 1110 manufactured by Daicel FineChem Ltd. (3 parts byweight), isopropyl alcohol (51.6 parts by weight), pure water (292 partsby weight) and an oxide (α alumina) powder as a reference (100 parts byweight) were mixed in this order, followed by stirring. After ultrasonicdispersion for 10 minutes, circulation dispersion was performed for 21minutes using Creamix (“CLM-0.8S”, manufactured by M Technique Co.,Ltd.), and then the thus obtained dispersion was filtered through a netwith 10 μm mesh size to prepare a slurry.

Subsequently, using a bar coater (#20), the slurry was applied on thebase material porous film and dried at a drying temperature of 65° C. toobtain a laminated porous film for evaluation in which an inorganicoxide porous film is formed on a surface of the base material porousfilm.

(Slurry Viscosity)

Using “TVB10M” manufactured by TOKI SANGYO CO., LTD. as a viscometer ofa slurry used in the case of forming the laminated porous film forevaluation, the measurement was performed by rotating a No. 3 rotor at 6rpm.

(Pore Diameter Inside Coating Film, Pore Volume Inside Coating Film, andPorosity)

A laminated porous film for evaluation was impregnated with an epoxyresin, followed by curing. After a cured sample was fixed to a samplestage and the FIB processing was applied to the sample by FIB-SEM[(HELIOS600) manufactured by FEI] to form a cross section, and the crosssection (surface of the inorganic oxide porous film) was observed by SEMat an accelerating voltage of 2.1 kV. After observation, the FIBprocessing was again applied to the sample with a thickness of 20 nm ina depth direction (film thickness direction of the inorganic oxideporous film) of the sample to form a new cross section, and the crosssection was observed by SEM. In this manner, the FIB processing and SEMobservation of the cross section were repeated at intervals of 20 nm toobtain continuous slice images including the total thickness of theinorganic oxide porous film. A position correction was made by the imageanalysis software (Avizo ver. 6.0 manufactured by Visualization SciencesGroup) to obtain continuous slice images. The scale was set to 10.4nm/pix for X-axis and Y-axis, and 20 nm/pix for Z-axis.

Using quantitative analysis software TRT/3D-BON (manufactured by RATOCSYSTEM ENGINEERNING CO., LTD.), three-dimensional analysis of thecoating film was applied to the obtained continuous slice images tocalculate a pore diameter inside coating film, a pore volume insidecoating film and porosity.

The three-dimensional quantitative analysis is made as follows. That is,files of the continuous slice images were firstly opened on TRI/3D-BONand a median filter (3D, 3×3) was applied, and then the particle portionand the void portion were distinguished by binarization using Auto-LW.

With respect to the void portion distinguished by the above processing,noises were removed under the conditions of 2D Ers Sml=1 and 3D ErsSml=5. Then, the value of the Thickness parameter was subjected to acalculation processing under the conditions of MIL=0.5, NdNd=1.5 andNdTm=2.0. The minor diameter Thickness, the major diameter Width, thedistance between branching points Length, the total volume BV of thevoid portion and the total volume TV of an analysis region weredetermined to define pores inside coating film, and then a pore diameterinside coating film, a pore volume inside coating film, and a porositywere calculated from the above formulas (3), (5), and (6).

(Volume Proportion of Pores Having Pore Diameter of 0.2 μm or Less)

Pore distribution inside coating film was determined from the porediameter inside coating film and the pore volume inside coating filmobtained by the above method. Among pores inside coating film, thevolume proportion of pores having a pore diameter of 0.2 μm or less(“total of a pore volume of pores having a pore diameter of 0.2 μm orless”/“total of a pore volume of all pores inside coating film”) wascalculated. In the case of this pore distribution inside coating film(volume proportion of pores having a pore diameter of 0.2 μm or less), aregion of 17.6 μm×11.3 μm×4.8 μm (954.6 μm³) was set as the measurementrange.

(Coating Film Thickness of Inorganic Oxide Porous Film)

The thickness (unit: μm) was measured by a high-accuracy digitalmeasuring instrument “VL-50A” manufactured by Mitutoyo Corporation. Acoating film thickness of the inorganic oxide porous film was calculatedby subtracting the thickness of the base material porous film from thethickness of the laminated porous film.

(Basis Weight of Inorganic Oxide Porous Film)

The laminated porous film was cut into pieces measuring 8 cm×8 cm andthe weight W (g) was measured, and then a basis weight (g/m²) of alaminated porous film=W/(0.08×0.08) was calculated.

The basis weight of the inorganic oxide porous film was calculated bysubtracting a basis weight of the base material porous film.

(Heat Shape Retention Ratio)

The laminated porous film was cut into pieces measuring 8 cm×8 cm, andthe cut out film on which a tetragon measuring 6 cm×6 cm was drawn wasinterposed between papers, followed by putting in an oven heated at 150°C. After one hour, the film was taken out from the oven and the size ofthe side of the tetragon drawn on the film was measured, and then a heatshape retention ratio was calculated. The calculation method is asfollows.

Length of line drawn in MD direction before heating: L1

Length of line drawn in MD direction after heating: L2

MD heat shape retention ratio (%)=(L2/L1)×100

Each of L1 and L2 was an average of both right and left sides in MDdirection of a drawn square. MD direction as used herein means a longdirection during forming of the base material porous film sheet.

(Air Permeability)

In accordance with JIS P8117(2009), Gurley value of the laminated porousfilm was measured by a Gurley type densometer manufactured by Toyo SeikiSeisaku-Sho, Ltd.

(Powder Falling Properties of Inorganic Oxide Porous Film (PowderFalling Ratio))

The measurement was performed in a surface rubbing test using areciprocating abrasion tester “TRIBOGEAR TYPE: 30” manufactured byShinto Scientific Co., Ltd. One white cloth (Kanakin No. 3) was attachedto a rubbing portion of the reciprocating abrasion tester and broughtinto contact with the inorganic oxide porous film side of the laminatedinorganic porous film under a load of 50 g/m². After performing 100reciprocations at a rate of 6,000 mm/minute (50 mm stroke) in an MDdirection, the weight B (g) of the inorganic oxide porous film existingin the rubbing portion was calculated from a basis weight (g/m²) of theabove-mentioned inorganic oxide porous film and the total area (m²) ofthe rubbing portion, and then a powder falling ratio (% by weight) wasdetermined from the weight of the laminated porous film before and afterthe reciprocating abration test using the following formula. It can besaid that lower powder falling ratio (% by weight) leads to higher filmstrength.

Powder falling ratio (% by weight)={(film weight before reciprocatingabrasion)−(film weight after reciprocating abrasion)}/B×100

Example 1

First, aluminum isopropoxide prepared from aluminum having a purity of99.99% as a raw material was hydrolyzed with water to obtain aslurry-like aluminum hydroxide, and then dried to obtain a dry-powderedaluminum hydroxide having an untamped density of 0.1 g/cm³.

Furthermore, this dry-powdered aluminum hydroxide was calcined byholding in an electric furnace under an air atmosphere at 1,200° C. for2.5 hours, and the agglomerated particles were crushed by a jet mill toobtain an α alumina powder (1).

Each content of impurities of the thus obtained α alumina powder (1) wasas follows: Si=8 ppm, Fe=31 ppm, Cu=1 ppm or less, Na=2 ppm, Mg=1 ppm orless and Zr=10 ppm or less, and oxide purity was 99.99% by weight ormore on an alumina basis. The BET specific surface area was 7.5 m²/g,the average three-dimensional particle unevenness of 100 or moreparticles by FIB-SEM was 5.0, and the abundance ratio in number ofparticles having a particle diameter of less than 0.3 μm was 77.0%.

Furthermore, an α alumina slurry was prepared from the α alumina powder(1) by the above-mentioned method, as a result, the viscosity was 48mPa·s. This slurry was applied on a base material porous film to form alaminated porous film for evaluation in which an inorganic oxide porousfilm is formed on a surface. The porosity of the inorganic oxide porousfilm by FIB-SEM was 52.0%, and pore distribution inside coating film(volume proportion of pores having a pore diameter of 0.2 μm or less)was 61.9%. The heat shape retention ratio of the thus obtained laminatedporous film was 93.0%. In addition, the evaluation results of coatingfilm thickness, basis weight, air permeability, powder fallingproperties and the like are shown in Tables 1 and 2. The inorganic oxideporous film thus obtained has sufficient porosity to ion permeation,pore diameter inside coating film, air permeability, high heatresistance and film strength. Therefore, it is apparent that, when usingthis inorganic oxide powder, a nonaqueous electrolyte secondary batteryhaving good battery performance regardless of a small basis weight, andalso having high safety is obtained.

Comparative Example 1

The same operation as in Example 1 was performed, except for using an αalumina powder obtained by changing only calcination conditions toconditions of holding in a gas furnace in which calcination wasperformed by combustion of a propane gas at 1,220° C. for 4 hours, inplace of the α alumina powder (1) obtained in Example 1, and an αalumina powder (2) was obtained.

Each content of impurities of the thus obtained α alumina powder (2) wasas follows: Si=5 ppm, Fe=4 ppm, Cu=1 ppm or less, Na=2 ppm, Mg=1 ppmZr=10 ppm or less, and oxide purity was 99.99% by weight or more on analumina basis. The BET specific surface area was 4.4 m²/g, the averagethree-dimensional particle unevenness of 100 or more particles byFIB-SEM was 4.4, and the abundance ratio in number of particles having aparticle diameter of less than 0.3 μm was 25.1%.

Furthermore, the viscosity of a slurry prepared from the α aluminapowder (2) by the above-mentioned method was 53 mPa·s. This slurry wasapplied on a base material porous film to form a laminated porous filmfor evaluation in which an inorganic oxide porous film is formed on asurface. The porosity of the inorganic oxide porous film by FIB-SEM was54%, and pore distribution inside coating film (volume proportion ofpores having a pore diameter of 0.2 μm or less) was 21.4%. The heatshape retention ratio of the thus obtained laminated porous film was32%. In addition, the evaluation results of coating film thickness,basis weight, air permeability, powder falling properties and the likeare shown in Tables 1 and 2.

TABLE 1 Average Abundance ratio in three- number of BET dimensionalparticles having a specific particle particle diameter surface Coatingunevenness of less than 0.3 μm area solution (—) (%) (m²/g) Example 1Alpha (α) 5.0 77.0 7.5 alumina powder (1) Comparative Alpha (α) 4.4 25.14.4 Example 1 alumina powder (2)

TABLE 2 Volume proportion MD heat Thickness of pores having a shapePowder of coating pore diameter of retention Basis Air falling filmPorosity 0.2 μm or less ratio weight permeability ratio (μm) (%) (%) (%)(g/m²) (sec/100 cc) (%) Example 1 5.0 52.0 61.9 93.0 8.7 207.0 55.0Comparative 5.9 54.0 21.4 32.0 9.1 226.0 80.0 Example 1

The present application claims priority on Japanese Patent ApplicationNo. 2014-255465 filed on Dec. 17, 2014, the disclosure of which isincorporated by reference herein.

INDUSTRIAL APPLICABILITY

The inorganic oxide powder of the present invention can provide aninorganic oxide porous film for use in a nonaqueous electrolytesecondary battery, which is excellent in ion conductivity, and also hashigh porosity, high film strength and heat resistance. The inorganicoxide porous film is excellent in ion conductivity and heat resistance,regardless of a small basis weight, and a nonaqueous electrolytesecondary battery including the inorganic oxide porous film on at leastone surface of a positive electrode, a negative electrode, or aseparator is industrially promising since the secondary battery thusobtained is excellent in battery performance and safety.

What is claimed is:
 1. An inorganic oxide porous film, wherein aproportion of a total of a pore volume inside coating film of poresinside coating film having a pore diameter inside coating film of 0.2 μmor less, relative to a total of a pore volume inside coating film of allpores inside coating film, is 35% or more.
 2. The inorganic oxide porousfilm according to claim 1, wherein the inorganic oxide porous filmcomprises an inorganic oxide powder having: 1) an averagethree-dimensional particle unevenness of 3.6 or more, and 2) anabundance ratio in number of particles having a particle diameter ofless than 0.3 μm of 50% or more.
 3. The inorganic oxide porous filmaccording to claim 1, wherein the inorganic oxide is α alumina.
 4. Theinorganic oxide porous film according to claim 2, wherein the inorganicoxide powder has a BET specific surface area of 6.0 m²/g or more.
 5. Theinorganic oxide porous film according to claim 1, wherein the inorganicoxide porous film has a porosity of a range of 30 to 75%.
 6. A laminate,wherein: the inorganic oxide porous film according to claim 1 is formedon a surface of a positive electrode, a negative electrode or aseparator.
 7. A nonaqueous electrolyte secondary battery, wherein: theinorganic oxide porous film according to claim 1 is formed on at leastone surface of a positive electrode, a negative electrode or aseparator.
 8. A laminated porous film, wherein: the inorganic oxideporous film according to claim 1 is formed on a base material porousfilm.